you can use doom.exe +set win_allowMultipleInstances 1 or add +set win_allowMultipleInstances 1 in a file named autoexec.cfg ... or in a text file like adisark.cfg and launch doom.exe with +exec adisark.cfg

you can use all cvars ...some are in doomconfig.cfg like allowconsole that you should use...lauching multiple doom is mostly good when you use internal editor : type editor in console to discover it